Samsung and Seagate are working on so-called hybrid drives employing both flash memory and platters to store data. It seems that the flash would be used as a cache to hold data before writing it to the hard drive and also for holding data that needs to be sent to the attached computer's memory either often or fast.
The advantage of having a few GB of SSD (solid state disk) inside an HDD (hard disk drive) casing is that you can store quite a lot of data on it and so optimise HDD operation.
You can re-order the data for faster writing with fewer head movements. You can also use the flash as a buffer and only spin up the hard drive when you need to send data to it or send data to it once it has been spun up for reading. In other words the drive does not spin all the time.
This is similar in some ways to Copan's MAID concept where disks are spun down unless needed. It's is reckoned that such a technique could might extend notebook computer battery life by up to an hour.
However, suppose the notebook computer's hybrid disk drive is spun down and there is a read request. Then the HDD needs spinning up before it can be satisfied and the read request takes longer to satisfy. Where read requests are random in nature then trying to predict them will defeat most algorithms.
It is not clear if this spin-the-drive-down concept will actually work in real life. Will users actually mind the extended read time likely to result from cache misses for requested data?
The flash memory in a hybrid drive could be used to store applications loaded during the boot sequence. This could shave minutes off boot time. It requires co-operation from the operating system so that it could direct which applications are held in cache. Should it store them as well as or instead of storing them on the HDD? Safety might indicate a dual copy storage policy. But flash retains data when power is switched off, like a hard drive so people who wish to maximise capacity might wish to only have a single copy policy.
Once the computer is started up then reading data that is in the hybrid drive's cache will be much faster than getting it off the HDD side of the combined drive.
But a cache miss will seem commensurately slower then before unless the HDD is continually spinning. Perhaps it will be an O/S option? You might set your notebook's hybrid drive policy to emphasise read speed - in which case the drive spins all the time - or focus on battery life - in which case the drive is spun down between read requests.
Such hybrid drives are a point along the way to a full SSD for notebook computers. But that will have to wait until 30-40GB SSDs become affordable. When that happens then all three factors: read speed; write speed; and battery life, will be greatly improved.